1. Field of Invention
The present invention relates to motion measurement; and more particularly relates to a method and system for measuring motion of an airborne platform.
While the invention may be subject to several applications, it is especially suited for use in a surveillance system for a tethered aerostat, and will be particularly described in that connection.
2. Description of Related Art
A tethered aerostat, or aerodynamic balloon, has proven to be a reliable and cost effective platform for wide area surveillance using state-of-the-art sensors. Aerostats, such as that utilized by a low altitude surveillance system can support substantial payloads to in the neighborhood of 15,000 feet above sea level. These fixed site systems are strategically located and are tethered to supporting ground mooring systems via a power tether which provides on station mission capabilities in the neighborhood of two weeks, for example.
The moored aerostat wanders about a circle of uncertainty of up to 1.5 nm about the mooring system. Of course, the actual location of the aerostat is a function of speed and direction of the winds aloft.
Early aerostat systems were primarily for air surveillance within a defined air space; and included a single low altitude surveillance system, and thus accuracy requirements were only modest. Target bearing measurements could be satisfied with directional gyroscopes slaved to magnetic, or in other words, flux gate sensors, for north referencing. Ground control intercept was within the coordinate system of the singular surveillance system only; and thus absolute geographic reference was not critical, even though the aerostat carried payload could be displaced relative to the mooring point by as much as 1.5 nm, under high wind blow down conditions.
Previously, motion measurement systems used a flux gate referenced directional gyro to indicate aerostat pointing angle relative to north. Antenna pointing angle relative to the aerostat was then determined by adding the antenna angle relative to the aerostat space angle by passing the directional gyro synchro signal through a differential transformer mounted to the payload azimuth drive unit. In this configuration, the directional gyro was mounted to the aerostat super rack forward of the payload truss and radar pedestal. This created two error sources. The directional gyro was essentially mounted to the aerostat, and directly experienced any aerostat pitch and roll motion. Since a directional gyro is typically a two degree of freedom device, this induced predictable yaw measurement errors, called non-verticality or pendulous errors, and which are trigonometric functions of the pitch and roll components. For a possible aerostat pitch and roll of .+-.10.degree., yaw error could be as high as .+-.1.75.degree. or 0.6.degree.; root mean square (RMS), for example. Secondly, the super rack location introduced a flexible structure error component between the gyro and radar pedestal. Both of these errors are in evidence under turbulent conditions.
Subsequently, the directional gyro was located directly on the radar payload pedestal, on the gravity stabilized side of the viscous damped gimbal system, but not on the rotating payload platform. This configuration essentially eliminated the unknown flexure of the gyro-to-pedestal and the non-verticality error; and platform pitch and roll was reduced typically to less than .+-.1.degree. which translates to a non-verticality error of .+-.0.017.degree. or 0.006.degree. RMS. This configuration, therefore, obviated the need for a three degree of freedom azimuth measuring device. Although payload sensor (radar and beacon) azimuth report accuracies have been measured at levels expected of similar ground based sensors, during times of aerostat motion, the scan to scan azimuth accuracies have been shown to be degraded by objectionable systematic error components. This was evidenced by several low frequency components and has been referred to by display operators as target stitching.
Error sources were speculated to be due to coupling of the magnetic flux gate into the gyro outputs as the aerostat was subjected to turbulent conditions. This was likely due to pendulous errors of the flux gate itself, as it was mounted in an unstabilized location on the aerostat, or due to non-compensation of the flux gate, and changes in local magnetic fields aboard the aerostat, as wind direction shifted. Attempts to calibrate the flux gate with techniques successfully used on aircraft installations were unsuccessful because of the large ferrous components of the aerostat mooring system nearby.
In many respects, an aerostat is a rather benign environment, as compared to a commercial or military aircraft for which inertial systems are designed. However, absolute north reference of a relatively stable system for as long as two weeks, for example, which is required for accurate surveillance, proved to be a problem.
A measurement system for determining continuously the actual latitude and longitude of targets requires an inertial navigation system, utilizing gyros, which are typically slaved to some north reference device for long term stability. Typically, the gyros align to north while in a non-moving ground environment. Then, of course, they must be updated along the flight path by external inputs, such as from a global positioning system (GPS) or Loran C for example. The gyros of an inertial navigation system may be either, the well known mechanical gyros or Ring Laser gyros, for example. However, such inertial navigation units are considered unacceptable for tethered aerostats for several reasons. The long term performance of north referencing beyond approximately eighteen hours cannot be assured. An inertial navigation system can not typically be realigned in-flight with the aerostat pitching and rolling.
Additionally, the netting of several low altitude surveillance radar systems and beacons mounted on multiple aerostats, and with corresponding multiple ground stations is required. The netting requirements impose a relative stringent geographically referenced azimuth accuracy requirement, as well as a scan-to-scan repeatability requirement necessary to address the "target stitching" phenomena to meet overall system accuracy requirements. Furthermore, a tethered aerostat experiences translational motion in turbulent conditions which can approach 100 feet per second. Doppler based sensors, such as radar, must also be compensated for this aerostat motion along the sensor line of sight. Previous inertial navigation systems can not provide translational velocity measurements to the required accuracy of 0.5 feet per second or better, over extended mission times of aerostats, without periodic position updating.
In light of the foregoing, there is a need for reliable motion measurement of an airborne platform that is capable of both long term and short term measurement accuracy, which can provide scan to scan azimuth angle repeatibility, which can provide line of sight sensor velocity, and is able to provide an accurate geographically stabilized sensor bearing measurement continuously without regard to atmospheric conditions; and still can be fabricated of components of medium precision and lower cost, as compared to high precision costly components.